Ontogenic changes of the control by phosphodiesterase-3 and -4 of 5-HT responses in porcine heart and relevance to human atrial 5-HT(4) receptors

Cardiac Roles of Serotonin (5-HT) and 5-HT-Receptors in Health and Disease

Serotonin acts solely via 5-HT₄-receptors to control human cardiac contractile function. The effects of serotonin via 5-HT₄-receptors lead to positive inotropic and chronotropic effects, as well as arrhythmias, in the human heart. In addition, 5-HT₄-receptors may play a role in sepsis, ischaemia, and reperfusion. These presumptive effects of 5-HT₄-receptors are the focus of the present review. We also discuss the formation and inactivation of serotonin in the body, namely, in the heart. We identify cardiovascular diseases where serotonin might play a causative or additional role. We address the mechanisms which 5-HT₄-receptors can use for cardiac signal transduction and their possible roles in cardiac diseases. We define areas where further research in this regard should be directed in the future, and identify animal models that might be generated to this end. Finally, we discuss in what regard 5-HT₄-receptor agonists or antagonists might be useful drugs that could enter clinical practice. Serotonin has been the target of many studies for decades; thus, we found it timely to summarise our current knowledge here.

Functional interaction of H2-receptors and 5HT4-receptors in atrial tissues isolated from double transgenic mice and from human patients

In the past, we generated transgenic mice that overexpress the human histamine 2 (H₂)-receptor (H₂-TG) or that overexpress the human serotonin 4 (5-HT₄)-receptor (5-HT₄-TG) in the heart. Here, we crossbred these lines of mice to generate double transgenic mice that overexpress both receptors (DT). This was done to study a conceivable interaction between these receptors in the mouse heart as a model for the human heart. When in left atria, initially, force of contraction was elevated maximally with 1 µM serotonin, and subsequently, histamine was cumulatively applied; a biphasic effect of histamine was noted: the force of contraction initially decreased, maximally at 10 nM histamine, and thereafter, the force of contraction increased again at 1 µM histamine. Notably, functional interaction between 5-HT and histamine was also identified in isolated electrically stimulated trabeculae carneae from human right atrium (obtained during cardiac surgery). These functional and biochemical data together are consistent with a joint overexpression of inotropically active H₂-receptors and 5-HT₄-receptors in the same mouse heart. We also describe an antagonistic interaction on the force of contraction of both receptors in the mouse atrium (DT) and in the human atrial muscle strips. We speculate that via this interaction, histamine might act as a "brake" on the cardiac actions of 5-HT via inhibitory GTP-binding proteins acting on the activity of adenylyl cyclase.

Molecular Basis of Atrial Fibrillation Initiation and Maintenance

Atrial fibrillation (AF) is the most common cardiac arrhythmia, largely associated to morbidity and mortality. Over the past decades, research in appearance and progression of this arrhythmia have turned into significant advances in its management. However, the incidence of AF continues to increase with the aging of the population and many important fundamental and translational underlaying mechanisms remain elusive. Here, we review recent advances in molecular and cellular basis for AF initiation, maintenance and progression. We first provide an overview of the basic molecular and electrophysiological mechanisms that lead and characterize AF. Next, we discuss the upstream regulatory factors conducting the underlying mechanisms which drive electrical and structural AF-associated remodeling, including genetic factors (risk variants associated to AF as transcriptional regulators and genetic changes associated to AF), neurohormonal regulation (i.e., cAMP) and oxidative stress imbalance (cGMP and mitochondrial dysfunction). Finally, we discuss the potential therapeutic implications of those findings, the knowledge gaps and consider future approaches to improve clinical management.

Regulation of basal and norepinephrine-induced cAMP and ICa in hiPSC-cardiomyocytes: Effects of culture conditions and comparison to adult human atrial cardiomyocytes

BACKGROUND

There is ongoing interest in generating cardiomyocytes derived from human induced pluripotent stem cells (hiPSC) to study human cardiac physiology and pathophysiology. Recently we found that norepinephrine-stimulated calcium currents (I_Ca) in hiPSC-cardiomyocytes were smaller in conventional monolayers (ML) than in engineered heart tissue (EHT). In order to elucidate culture specific regulation of β₁-adrenoceptor (β₁-AR) responses we investigated whether action of phosphodiesterases (PDEs) may limit norepinephrine effects on I_Ca and on cytosolic cAMP in hiPSC-cardiomyocytes. Results were compared to adult human atrial cardiomyocytes.

METHODS

Adult human atrial cardiomyocytes were isolated from tissue samples obtained during open heart surgery. All patients were in sinus rhythm. HiPSC-cardiomyocytes were dissociated from ML and EHT. Förster-resonance energy transfer (FRET) was used to monitor cytosolic cAMP (Epac1-camps sensor, transfected by adenovirus). I_Ca was recorded by whole-cell patch clamp technique. Cilostamide (300 nM) and rolipram (10 μM) were used to inhibit PDE3 and PDE4, respectively. β₁-AR were stimulated with the physiological agonist norepinephrine (100 μM).

RESULTS

In adult human atrial cardiomyocytes, norepinephrine increased cytosolic cAMP FRET ratio by +13.7 ± 1.5% (n = 10/9, mean ± SEM, number of cells/number patients) and I_Ca by +10.4 ± 1.5 pA/pF (n = 15/10). This effect was not further increased in the concomitant presence of rolipram, cilostamide and norepinephrine, indicating saturation by norepinephrine alone. In ML hiPSC-cardiomyocytes, norepinephrine exerted smaller increases in cytosolic cAMP and I_Ca (FRET +9.6 ± 0.5% n = 52/21, number of cells/number of ML or EHT, and I_Ca + 1.4 ± 0.2 pA/pF n = 34/7, p < 0.05 each) and both were augmented in the presence of the PDE4 inhibitor rolipram (FRET +16.7 ± 0.8% n = 94/26 and I_Ca + 5.6 ± 1.4 pA/pF n = 11/5, p < 0.05 each). Cilostamide increased the response to norepinephrine on FRET (+12.7 ± 0.5% n = 91/19, p < 0.05), but not on I_Ca. In EHT hiPSC-cardiomyocytes, norepinephrine responses on both, FRET and I_Ca, were larger than in ML (FRET +12.1 ± 0.3% n = 87/32 and I_Ca + 3.3 ± 0.2 pA/pF n = 13/5, p < 0.05 each). Rolipram augmented the norepinephrine effect on I_Ca (+6.2 ± 1.6 pA/pF; p < 0.05 vs. norepinephrine alone, n = 10/4), but not on FRET.

CONCLUSION

Our results show culture-dependent differences in hiPSC-cardiomyocytes. In conventional ML but not in EHT, maximum norepinephrine effects on cytosolic cAMP depend on PDE3 and PDE4, suggesting immaturity when compared to the situation in adult human atrial cardiomyocytes. The smaller I_Ca responses to norepinephrine in ML and EHT vs. adult human atrial cardiomyocytes depend at least partially on a non-physiological large impact of PDE4 in hiPSC-cardiomyocytes.

Phosphodiesterases 2, 3 and 4 can decrease cardiac effects of H2-histamine-receptor activation in isolated atria of transgenic mice

Histamine exerts cAMP-dependent positive inotropic effects (PIE) and positive chronotropic effects (PCE) on isolated left and right atria, respectively, of transgenic mice which overexpress the human H₂-receptor in the heart (=H₂-TG). To determine whether these effects are antagonized by phosphodiesterases (PDEs), contractile studies were done in isolated left and right atrial preparations of H₂-TG. The contractile effects of histamine were tested in the additional presence of the PDE-inhibitorserythro-9-(2-hydroxy-3-nonyl)adenine hydrochloride (EHNA, 1 μM, PDE2-inhibitor) or cilostamide (1 μM, PDE3-inhibitor), rolipram (10 μM, a PDE4-inhibitor), and their combinations. Cilostamide (1 μM) and EHNA (1 μM), rolipram (1 μM), and EHNA (1 μM) and the combination of rolipram (0.1 μM) and cilostamide (1 μM) each increased the potency of histamine to elevate the force of contraction (FOC) in H₂-TG. Cilostamide (1 μM) and rolipram (10 μM) alone increased and EHNA (1 μM) decreased alone, and their combination increased the potency of histamine to increase the FOC in H₂-TG indicating that PDE3 and PDE4 regulate the inotropic effects of histamine in H₂-TG. The PDE inhibitors (EHNA, cilostamide, rolipram) alone did not alter the potency of histamine to increase the heart beat in H₂-TG whereas a combination of rolipram, cilostamide, and EHNA, or of rolipram and EHNA increased the potency of histamine to act on the beating rate. In summary, the data suggest that the PCE of histamine in H₂-TG atrium involves PDE 2 and 4 activities, whereas the PIE of histamine are diminished by activity of PDE 3 and 4.

Impact of phosphodiesterases PDE3 and PDE4 on 5-hydroxytryptamine receptor4-mediated increase of cAMP in human atrial fibrillation

Atrial fibrillation (AF)-associated remodeling includes contractile dysfunction whose reasons are only partially resolved. Serotonin (5-HT) increases contractile force and causes arrhythmias in atrial trabeculae from patients in sinus rhythm (SR). In persistent atrial fibrillation (peAF), the force responses to 5-HT are blunted and arrhythmic effects are abolished. Since force but not arrhythmic responses to 5-HT in peAF could be restored by PDE3 + PDE4 inhibition, we sought to perform real-time measurements of cAMP to understand whether peAF alters PDE3 + PDE4-mediated compartmentation of 5-HT₄ receptor-cAMP responses. Isolated human atrial myocytes from patients in SR, with paroxysmal AF (paAF) or peAF, were adenovirally transduced to express the FRET-based cAMP sensor Epac1-camps. Forty-eight hours later, cAMP responses to 5-HT (100 μM) were measured in the absence or concomitant presence of the PDE3 inhibitor cilostamide (0.3 μM) and the PDE4 inhibitor rolipram (1 μM). We successfully established real-time cAMP imaging in AF myocytes. 5-HT increased cAMP in SR, paAF, and peAF, but in line with previous findings on contractility, this increase was considerably smaller in peAF than in SR or paAF. The maximal cAMP response to forskolin (10 μM) was preserved in all groups. The diminished cAMP response to 5-HT in peAF was recovered by preincubation with cilostamide + rolipram. We uncovered a significantly diminished cAMP response to 5-HT₄ receptor stimulation which may explain the blunted 5-HT inotropic responses observed in peAF. Since both cAMP and force responses but not arrhythmic responses were recovered after concomitant inhibition of PDE3 + PDE4, they might be regulated in different subcellular microdomains.

Regulation of ICa,L and force by PDEs in human-induced pluripotent stem cell-derived cardiomyocytes

Background and Purpose

Phosphodiesterases (PDEs) are important regulators of β‐adrenoceptor signalling in the heart. While PDE4 is the most important isoform that regulates I_Ca,L and force in rodent cardiomyocytes, the dominant isoform in adult human cardiomyocytes is PDE3.

Experimental Approach

Given the potential of human‐induced pluripotent stem cell‐derived cardiomyocytes (hiPSC‐CMs) for biomedical research, this study characterized the contribution of PDE3 and PDE4 isoforms to the regulation of I_Ca,L and force in hiPSC‐CMs in an engineered heart tissue (EHT) model.

Key Results

There was a lower abundance of mRNA for PDE3A and 4A in hiPSC‐CM EHT than in non‐failing human heart samples. Selective inhibition of PDE3 and 4 with cilostamide and rolipram, respectively, showed that, in hiPSC‐CM, PDE4 was the predominant isoform for the regulation of I_Ca,L (cilostamide: +1.44‐fold; rolipram: +1.77‐fold)_. Furthermore, in contrast to cilostamide, rolipram decreased the EC₅₀ of isoprenaline about 15‐fold.

Conclusion and implications

The predominance of PDE4 over PDE3 is a peculiarity of hiPSC‐CMs and is probably an indicator of immaturity. This finding has implications for the use of hiPSC‐CM as pharmacological models to investigate and assess the effects of PDE inhibitors.

Effect of sepsis on the action potential and cardiac serotonin response in rats

The current study aimed to investigate the effect of sepsis on rat serotonin (5-HT) responses and cardiac action potentials. A total of 20 rats were randomly divided into a sepsis and control group (each, n=10). Rat hearts were harvested and perfused using the Langendorff method 18-h after the induction of sepsis, which was assessed using cecal puncture. Cardiac action potential was subsequently measured using a multichannel electrophysiology instrument. Immunohistochemistry and quantitative analysis were performed to identify the effect of sepsis on myocardial 5-HT expression. The results revealed that mitochondrial changes were present in septic rat hearts. Heart rate (361.10±12.29 bpm vs. 348.60±12.38 bpm; P<0.05) was significantly higher, atrial action potential duration (106.40±2.95 ms vs. 86.60±4.12 ms; P<0.01) was significantly longer and the area (0.62±0.06 µm² vs. 0.39±0.05 µm²; P<0.05) and number (0.92±0.02/field vs. 0.46±0.01/field; P<0.01) of myocardial cells were significantly higher in the septic compared with the control group. These results demonstrated that 5-HT prolongs the atrial action potential, increases heart rate and aggravates myocardial injury, indicating that it may therefore be one of the factors that leads to cardiac dysfunction in sepsis.

Synergic PDE3 and PDE4 control intracellular cAMP and cardiac excitation-contraction coupling in a porcine model

AIMS

Cyclic AMP phosphodiesterases (PDEs) are important modulators of the cardiac response to β-adrenergic receptor (β-AR) stimulation. PDE3 is classically considered as the major cardiac PDE in large mammals and human, while PDE4 is preponderant in rodents. However, it remains unclear whether PDE4 also plays a functional role in large mammals. Our purpose was to understand the role of PDE4 in cAMP hydrolysis and excitation-contraction coupling (ECC) in the pig heart, a relevant pre-clinical model.

METHODS AND RESULTS

Real-time cAMP variations were measured in isolated adult pig right ventricular myocytes (APVMs) using a Förster resonance energy transfer (FRET) biosensor. ECC was investigated in APVMs loaded with Fura-2 and paced at 1 Hz allowing simultaneous measurement of intracellular Ca²⁺ and sarcomere shortening. The expression of the different PDE4 subfamilies was assessed by Western blot in pig right ventricles and APVMs. Similarly to PDE3 inhibition with cilostamide (Cil), PDE4 inhibition with Ro 20-1724 (Ro) increased cAMP levels and inotropy under basal conditions. PDE4 inhibition enhanced the effects of the non-selective β-AR agonist isoprenaline (Iso) and the effects of Cil, and increased spontaneous diastolic Ca²⁺ waves (SCWs) in these conditions. PDE3A, PDE4A, PDE4B and PDE4D subfamilies are expressed in pig ventricles. In APVMs isolated from a porcine model of repaired tetralogy of Fallot which leads to right ventricular failure, PDE4 inhibition also exerts inotropic and pro-arrhythmic effects.

CONCLUSIONS

Our results show that PDE4 controls ECC in APVMs and suggest that PDE4 inhibitors exert inotropic and pro-arrhythmic effects upon PDE3 inhibition or β-AR stimulation in our pre-clinical model. Thus, PDE4 inhibitors should be used with caution in clinics as they may lead to arrhythmogenic events upon stress.

cAMP Catalyzing Phosphodiesterases Control Cholinergic Muscular Activity But Their Inhibition Does Not Enhance 5-HT4 Receptor-Mediated Facilitation of Cholinergic Contractions in the Murine Gastrointestinal Tract

Background: As the signal transduction of 5-HT₄ receptors on cholinergic neurons innervating smooth muscle is controlled by phosphodiesterase (PDE) 4 in porcine stomach and colon, and human large intestine, the in vivo gastroprokinetic effects of a 5-HT₄ receptor agonist might be enhanced by combination with a selective PDE4 inhibitor. The presence of 5-HT₄ receptors on cholinergic neurons towards murine gastrointestinal circular muscle was recently shown. If the control of this receptor pathway by PDE4 is also present in mice, this might be a good model for in vivo testing of the combination therapy. Therefore this study investigates the role of cAMP catalyzing PDEs in smooth muscle cell activity and in the intraneuronal signal transduction of the 5-HT₄ receptors in the gastrointestinal tract of C57Bl/6J mice.

Methods: In circular smooth muscle strips from murine fundus, jejunum, and colon, submaximal cholinergic contractions were induced by either electrical field stimulation (EFS) or by carbachol (muscarinic receptor agonist). The influence of the PDE inhibitors IBMX (non-selective), vinpocetine (PDE1), EHNA (PDE2), cilostamide (PDE3), and rolipram (PDE4) was tested on these contractions and on the facilitating effect of a submaximal concentration of prucalopride (5-HT₄ receptor agonist) on EFS-induced contractions.

Results: In the three gastrointestinal regions, IBMX and cilostamide concentration-dependently decreased carbachol- as well as EFS-induced contractions. Some inhibitory effect was also observed with rolipram. In the fundus a non-significant trend for an enhancement of the facilitating effect of prucalopride on EFS-induced contractions was observed with IBMX, but none of the selective PDE inhibitors enhanced the facilitating effect of prucalopride in fundus, jejunum or colon.

Conclusion: In analogy with the porcine gastrointestinal tract, in murine fundus, jejunum, and colon circular smooth muscle PDE3 is the main regulator of the cAMP turnover, with some contribution of PDE4. In contrast to the porcine gastrointestinal tract, the in vitro facilitation of electrically induced cholinergic contractions by 5-HT₄ receptor stimulation could not be enhanced by specific PDE inhibition. The C57Bl/6J murine model is thus not suitable for in vivo testing of a 5-HT₄ receptor agonist combined with a selective PDE4 inhibitor.

Synergy between 5-HT4 receptor stimulation and phosphodiesterase 4 inhibition in facilitating acetylcholine release in human large intestinal circular muscle

BACKGROUND

Gastroprokinetic properties of 5-HT₄ receptor agonists, such as prucalopride, are attributed to activation of 5-HT₄ receptors on cholinergic nerves innervating smooth muscle in the gastrointestinal smooth muscle layer, increasing acetylcholine release and muscle contraction. In porcine stomach and colon, phosphodiesterase (PDE) 4 has been shown to control the signaling pathway of these 5-HT₄ receptors. The aim of this study was to investigate the PDE-mediated control of these 5-HT₄ receptors in human large intestine.

METHODS

Circular smooth muscle strips were prepared from human large intestine; after incubation with [³H]-choline, electrically induced tritium outflow was determined as a measure for acetylcholine release. The influence of PDE inhibition on the facilitating effect of prucalopride on electrically induced acetylcholine release was studied.

KEY RESULTS

The non-selective PDE inhibitor IBMX enhanced the facilitating effect of prucalopride on electrically induced acetylcholine release. The selective inhibitors vinpocetine (PDE1), EHNA (PDE2) and cilostamide (PDE3) did not influence, while rolipram and roflumilast (PDE4) enhanced the prucalopride-induced facilitation to the same extent as IBMX.

CONCLUSIONS & INFERENCES

In human large intestinal circular muscle, the intracellular pathway of 5-HT₄ receptors facilitating cholinergic neurotransmission to large intestinal circular smooth muscle is controlled by PDE4. If the synergy between 5-HT₄ receptor agonism and PDE4 inhibition is confirmed in a functional assay with electrically induced cholinergic contractions of human large intestinal circular smooth muscle strips, combination of a selective 5-HT₄ receptor agonist with a selective PDE4 inhibitor might enhance the in vivo prokinetic effect of the 5-HT₄ receptor agonist in the large intestine.

Chronic Unpredictable Mild Stress Causing Cardiac and Thoracic Spinal Cord Electrophysiological Abnormalities May Be Associated with Increased Cardiac Expression of Serotonin and Growth-Associated Protein-43 in Rats

BACKGROUND

The aim of this study was to investigate the potential mechanisms by which chronic unpredictable mild stress (CMS) might induce cardiovascular disease.

METHODS

Twenty male Sprague-Dawley rats (weighing 180-250 g) were divided into the CMS group (CMS for 3 weeks) and control group (n = 10/group). Sucrose solution consumption, sucrose solution preference rate, and the open field test (horizontal and vertical movements) were used to confirm the establishment of the CMS model. Heart rate was determined in Langendorff-perfused hearts, and field action potential duration (FAPD) was measured in cardiac atrial tissue, cardiac ventricular tissue, and thoracic spinal cord segments 1-5. The expressions of serotonin (5-HT) and growth-associated protein-43 (GAP-43) in cardiac ventricular tissue were analyzed using immunohistochemistry and immunofluorescence.

RESULTS

Compared with the control group, sucrose solution consumption, sucrose solution preference rate, horizontal movement, and vertical movement were significantly lower in the CMS group (P < 0.01). The CMS group exhibited significant decreases in atrial and ventricular FAPDs (P < 0.05), as well as significant increases in heart rates (P < 0.05) and T1-5 spinal cord FAPD (P < 0.01), as compared with the control group. The expressions of 5-HT and GAP-43 in cardiac ventricular tissue were significantly higher in the CMS group than in controls (P < 0.01).

CONCLUSIONS

CMS causes cardiac and T1-5 spinal cord electrophysiological abnormalities as well as increased cardiac expression of 5-HT and GAP-43, indicating that CMS could potentially increase the risk of cardiovascular disease.

Desensitization of the human 5-HT4 receptor in isolated atria of transgenic mice

In the human cardiovascular system, serotonin (5-HT) exerts positive inotropic and chronotropic effects mediated by 5-HT₄ receptors. Moreover, 5-HT₄ receptor stimulation can cause arrhythmias in the human heart. Response to 5-HT can fade due to desensitization of the receptor system and/or activation of phosphodiesterases. In this study, we investigated a potential desensitization of the human 5-HT_4(a) receptor expressed in the mouse heart. Therefore, we have used atrial preparations of transgenic (TG) mice with cardiac myocyte-specific overexpression of the human 5-HT_4(a) receptor and their non-transgenic littermates (WT). Homologous (by 5-HT) and potentially heterologous (by isoprenaline) desensitization of the 5-HT₄ receptor was investigated in atria of TG mice. 5-HT increased force of contraction in isolated electrically paced left atria and beating rate in spontaneously beating right atria only in preparations from TG but not from WT. Pre-treatment of isolated atria with high concentrations (10-600 μM) of 5-HT for 60 min attenuated the positive inotropic effects and the positive chronotropic effects of 5-HT in TG atria. Several inhibitors of desensitization including Zn²⁺, sucrose, and paroxetine were tested. Whereas sucrose was without any effect and Zn²⁺ only was partially effective, paroxetine was able to inhibit desensitization favoring at least in part a G-protein receptor-coupled kinase-mediated mechanism of 5-HT₄ receptor desensitization in the TG mouse heart. In addition, desensitization of ventricular 5-HT₄ receptors was noted in isolated perfused hearts (Langendorff preparations) from TG mice. In summary, we show homologous desensitization of the 5-HT₄ receptor in the heart of a transgenic mouse model possibly dependent on active G-protein receptor-coupled kinase. The exact mechanism and a potentially heterologous desensitization have to be elucidated by further investigations.

The 5-HT4 receptor agonist prucalopride does not facilitate cholinergic neurotransmission in circular and longitudinal smooth muscle preparations of equine mid-jejunum

BACKGROUND

Postoperative ileus (POI) remains an important cause of death in horses. The recently developed selective 5-HT₄ receptor agonists such as prucalopride target 5-HT₄ receptors on myenteric cholinergic neurons to enhance acetylcholine release and GI motility. No clearcut in vitro evaluation whether highly selective 5-HT₄ receptor agonists enhance submaximal cholinergic neurotransmission towards the muscle layer has been performed in horses.

OBJECTIVES

To identify functional 5-HT₄ receptors in equine jejunum.

STUDY DESIGN

In vitro experimental study.

METHODS

Circular and longitudinal smooth muscle strips (mid-jejunum) were mounted in organ baths between 2 platinum electrodes allowing electrical field stimulation (EFS). To delineate the conditions to obtain purely cholinergic responses, voltage-response curves were studied. To investigate the influence of prucalopride and 5-HT, submaximal cholinergic contractions at a single voltage were induced.

RESULTS

In circular and longitudinal strips, EFS induced voltage-dependent neurogenic on-contractions when the bathing medium contained a NO-synthesis inhibitor and apamin to prevent inhibitory responses to NO and ATP. Contractions at a voltage inducing 50% of maximal amplitude were cholinergic, as they were blocked by atropine. These contractions were not influenced by prucalopride (up to 3μM), even in the presence of the phosphodiesterase inhibitor isobutyl-methyl-xanthine to inhibit breakdown of the second messenger of 5-HT₄ receptors, cAMP. Also the full 5-HT₄ receptor agonist 5-HT did not influence the EFS-induced submaximal cholinergic contractions. Moreover, prucalopride did not influence muscle tone continuously enhanced with KCl.

CONCLUSIONS

There are no functional 5-HT₄ receptors on myenteric cholinergic neurons nor muscular 5-HT₄ receptors in equine jejunum.

In permanent atrial fibrillation, PDE3 reduces force responses to 5-HT, but PDE3 and PDE4 do not cause the blunting of atrial arrhythmias

BACKGROUND AND PURPOSE

5-HT increases force and L-type Ca(2) (+) current (ICa,L ) and causes arrhythmias through 5-HT4 receptors in human atrium. In permanent atrial fibrillation (peAF), atrial force responses to 5-HT are blunted, arrhythmias abolished but ICa,L responses only moderately attenuated. We investigated whether, in peAF, this could be due to an increased function of PDE3 and/or PDE4, using the inhibitors cilostamide (300 nM) and rolipram (1 μM) respectively.

EXPERIMENTAL APPROACH

Contractile force, arrhythmic contractions and ICa,L were assessed in right atrial trabeculae and myocytes, obtained from patients with sinus rhythm (SR), paroxysmal atrial fibrillation (pAF) and peAF.

KEY RESULTS

Maximum force responses to 5-HT were reduced to 15% in peAF, but not in pAF. Cilostamide, but not rolipram, increased both the blunted force responses to 5-HT in peAF and the inotropic potency of 5-HT fourfold to sevenfold in trabeculae of patients with SR, pAF and peAF. Lusitropic responses to 5-HT were not decreased in peAF. Responses of ICa,L to 5-HT did not differ and were unaffected by cilostamide or rolipram in myocytes from patients with SR or peAF. Concurrent cilostamide and rolipram increased 5-HT's propensity to elicit arrhythmias in trabeculae from patients with SR, but not with peAF.

CONCLUSIONS AND IMPLICATIONS

PDE3, but not PDE4, reduced inotropic responses to 5-HT in peAF, independently of lusitropy and ICa,L , but PDE3 activity was the same as that in patients with SR and pAF. Atrial remodelling in peAF abolished the facilitation of 5-HT to induce arrhythmias by inhibition of PDE3 plus PDE4.

Synergistic effect between 5-HT4 receptor agonist and phosphodiesterase 4-inhibitor in releasing acetylcholine in pig gastric circular muscle in vitro

5-HT4 receptor agonists have a gastroprokinetic effect by facilitating acetylcholine release from cholinergic nerves innervating gastrointestinal smooth muscle. The role of phosphodiesterase (PDE) 4 in the signal transduction pathway of the 5-HT4 receptors located on the cholinergic neurons towards the circular muscle layer in pig stomach was investigated by analysis of acetylcholine release. Circular muscle strips were prepared from pig proximal stomach and tritium outflow, induced by electrical field stimulation, was studied as a marker for acetylcholine release after incubation with [(3)H]-choline. The PDE4-inhibitor roflumilast concentration-dependently (0.1-1µM) enhanced the facilitating effect of a submaximally effective concentration of the 5-HT4 receptor agonist prucalopride (0.01µM) on electrically induced acetylcholine release. Roflumilast (0.3µM) enhanced acetylcholine release per se but in the combined presence of roflumilast and prucalopride, acetylcholine release was enhanced more than the sum of the effect of the 2 compounds alone. The 5-HT4 receptor agonist velusetrag concentration-dependently (0.01-0.1µM) enhanced acetylcholine release; the effect of the minimally effective concentration (0.01µM) was significantly enhanced by 1µM of the PDE4-inhibitor rolipram, again to a level higher than the sum of the effect of the 2 compounds alone. The synergistic effect between 5-HT4 receptor agonists and PDE4-inhibitors demonstrates that the intracellular pathway of the 5-HT4 receptors located on cholinergic neurons towards pig gastric circular muscle is controlled by PDE4. Combining a 5-HT4 receptor agonist with a PDE4-inhibitor might thus enhance its gastroprokinetic effect.

Inhibitors of phosphodiesterases PDE2, PDE3, and PDE4 do not increase the sinoatrial tachycardia of noradrenaline and prostaglandin PGE₁ in mice

Phosphodiesterases PDE2, PDE3, and PDE4 are expressed in murine sinoatrial cells. PDE3 and/or PDE4 reduce heart rate but apparently do not influence the tachycardia mediated through sinoatrial β1- and β2-adrenoceptors despite the high content of sinoatrial cAMP. The function of PDE2 is, however, uncertain. Prostaglandin PGE1 elicits sinoatrial tachycardia through EP receptors, but the control by phosphodiesterases is unknown. We investigated on spontaneously beating right atria of mice the effects of the PDE2 inhibitors Bay 60-7550 and EHNA on basal beating and the tachycardia produced by noradrenaline (3 nM) and PGE1 (1 μM). Bay 60-7550 (1 μM), but not EHNA (10 μM), increased basal sinoatrial beating. EHNA also failed to produce tachycardia in the presence of the adenosine deaminase inhibitor 2'-deoxycoformycin (10 μM), remaining inconclusive whether PDE2 reduces basal sinoatrial beating. Rolipram (10 μM) and cilostamide (300 nM) caused moderate tachycardia. The tachycardia evoked by Bay 60-7550 was similar in the absence and presence of rolipram. Noradrenaline elicited stable tachycardia that was not increased by Bay 60-7550. A stable tachycardia caused by PGE1 was not increased by the inhibitors of PDE2, PDE3, and PDE4. Unlike PDE3 and PDE4 which reduce murine basal sinoatrial beating, a possible effect of PDE2 needs further research. The stable tachycardia produced by noradrenaline and PGE1, together with the lack potentiation by the inhibitors of PDE2, PDE3, and PDE4, suggests that cAMP generated at the receptor compartments is hardly hydrolyzed by these phophodiesterases. Evidence from human volunteers is consistent with this proposal.

Phosphodiesterase 4 interacts with the 5-HT4(b) receptor to regulate cAMP signaling

Phosphodiesterase (PDE) 3 and PDE4, which degrade cyclic adenosine monophosphate (cAMP), are important regulators of 5-hydroxytryptamine (5-HT) 4 receptor signaling in cardiac tissue. Therefore, we investigated whether they interact with the 5-HT4(b) receptor, and whether A-kinase anchoring proteins (AKAPs), scaffolding proteins that bind to the regulatory subunit of protein kinase A (PKA) and contribute to the spacial-temporal control of cAMP signaling, are involved in the regulation of 5-HT4(b) receptor signaling. By measuring PKA activity in the absence and presence of PDE3 and PDE4 inhibitiors, we found that constitutive signaling of the overexpressed HA-tagged 5-HT4(b) receptor in HEK293 cells is regulated predominantly by PDE4, with a secondary role for PDE3 that is unmasked in the presence of PDE4 inhibition. Overexpressed PDE4D3 and PDE3A1, and to a smaller extent PDE4D5 co-immunoprecipitate constitutively with the 5-HT4(b) receptor. PDE activity measurements in immunoprecipitates of the 5-HT4(b) receptor confirm the association of PDE4D3 with the receptor and provide evidence that the activity of this PDE may be increased upon receptor stimulation with 5-HT. A possible involvement of AKAPs in 5-HT4(b) receptor signaling was uncovered in experiments using the St-Ht31 inhibitor peptide, which disrupts the interaction of AKAPs with PKA. However, St-Ht31 did not influence 5-HT4(b) receptor-stimulated PKA activity, and endogenous AKAP79 and gravin were not found in immunoprecipitates of the 5-HT4(b) receptor. In conclusion, we found that both PDE3A1 and PDE4D3 are integrated into complexes that contain the 5-HT4(b) receptor and may thereby regulate 5-HT4(b) receptor-mediated signaling.

Cardiac PDEs and crosstalk between cAMP and cGMP signalling pathways in the regulation of contractility

Elucidation of cAMP and cGMP signalling in the heart remains a hot topic, and new regulatory mechanisms continue to appear. Studying the influence of phosphodiesterases on 5-HT4 receptor signalling in porcine atrium, a paper from this issue of the journal expands findings of a crosstalk between cardiac cGMP and cAMP signalling recently discovered in failing rat ventricle to a different species and cardiac region. The overall data suggest that cGMP, produced following stimulation of the NPR-B receptor for C-type natriuretic peptide (CNP), inhibits cAMP degradation by phosphodiesterase 3 and thereby enhances cAMP-mediated signalling from β-adrenoceptors and 5-HT4 receptors to inotropic effects. In porcine atrium, this effect can be seen both as an increase in inotropic effect and as a reduced fade of the inotropic effect with time. Thus, accumulating evidence brings together several active fields of research, including cardiac phosphodiesterases, compartmentation of cyclic nucleotide signalling and the field of natriuretic peptides. If present in human hearts, this effect of CNP may have clinical implications.

Influence of phosphodiesterases and cGMP on cAMP generation and on phosphorylation of phospholamban and troponin I by 5-HT4 receptor activation in porcine left atrium

Our objective was to investigate the role of phosphodiesterase (PDE)3 and PDE4 and cGMP in the control of cAMP metabolism and of phosphorylation of troponin I (TnI) and phospholamban (PLB) when 5-HT4 receptors are activated in pig left atrium. Electrically paced porcine left atrial muscles, mounted in organ baths, received stimulators of particulate guanylyl cyclase (pGC) or soluble guanylyl cyclase (sGC) and/or specific PDE inhibitors followed by 5-HT or the 5-HT4 receptor agonist prucalopride. Muscles were freeze-clamped at different moments of exposure to measure phosphorylation of the cAMP/protein kinase A targets TnI and PLB by immunoblotting and cAMP levels by enzyme immunoassay. Corresponding with the functional results, 5-HT only transiently increased cAMP content, but caused a less quickly declining phosphorylation of PLB and did not significantly change TnI phosphorylation. Under combined PDE3 and PDE4 inhibition, the 5-HT-induced increase in cAMP levels and PLB phosphorylation was enhanced and sustained, and TnI phosphorylation was now also increased. Responses to prucalopride per se and the influence thereupon of PDE3 and PDE4 inhibition were similar except that responses were generally smaller. Stimulation of pGC together with PDE4 inhibition increased 5-HT-induced PLB phosphorylation compared to 5-HT alone, consistent with functional responses. sGC stimulation hastened the fade of inotropic responses to 5-HT, while cAMP levels were not altered. PDE3 and PDE4 control the cAMP response to 5-HT4 receptor activation, causing a dampening of downstream signalling. Stimulation of pGC is able to enhance inotropic responses to 5-HT by increasing cAMP levels, while sGC stimulation decreases contraction to 5-HT cAMP independently.

Human 5-HT₄receptor stimulation in atria of transgenic mice

In human atrium, serotonin (5-HT) exerts pleiotropic effects, which are thought to be mediated via 5-HT4 receptors. Here, we used transgenic mice (TG) that overexpress the human 5-HT4(a) receptor under control of the heart-specific α-myosin heavy chain promoter in the atria (and ventricles). Contractile studies were performed in isolated electrically driven left atrial preparations and spontaneously beating right atrial preparation of TG and littermate control mice (wild type (WT)). 5-HT increased force of contraction and phospholamban phosphorylation on serine 16 only in left atrial preparations from TG but not from WT. In contrast, β-adrenoceptor stimulation of left atrial preparations by isoprenaline increased force of contraction with similar pEC50 values and to a similar maximum extent in both TG and WT. The contractile effects of 5-HT in left atrial preparations from TG could be blocked by the 5-HT4 receptor-specific antagonists GR125487 or GR113808. In right atrial preparations from WT and TG, the β-adrenoceptor agonist isoprenaline exerted a positive chronotropic effect with similar pEC50 values and similar maximum effects. Only in right atrial preparations from TG but not WT, 5-HT exerted a positive chronotropic effect that could be attenuated by 5-HT4 receptor-specific antagonists. Finally, in left atrial preparations of TG, a higher incidence of arrhythmias was noted compared to WT. The present data indicate that the human 5-HT4 receptors expressed in mouse atria are functional. This is the first transgenic model to study this human receptor in the atrium ex vivo or in vivo.

Study of the regulation of the inotropic response to 5-HT4 receptor activation via phosphodiesterases and its cross-talk with C-type natriuretic peptide in porcine left atrium

We studied how 5-HT(4) receptor-mediated inotropic responses are regulated at the level of cAMP in porcine left atrium. We used selective phosphodiesterase (PDE) inhibitors to assess which PDE subtypes are responsible for the fade with time of inotropic responses to 5-HT(4) receptor activation with 5-HT and the 5-HT(4) receptor agonist prucalopride. A possible cross-talk via PDEs between cGMP and 5-HT(4) receptor-induced cAMP signalling was evaluated. Electrically paced left atrial pectinate muscles from young male pigs (15-25 kg) were studied in vitro. Simultaneous inhibition of PDE3 plus PDE4 subtypes was necessary to increase the amplitude and completely prevent the fade of the inotropic response to 5-HT and prucalopride. When responses to 5-HT or prucalopride had faded 1 h after addition, the nonspecific PDE-inhibitor IBMX still fully recovered inotropic responses. Stimulation of particulate guanylyl cyclase, together with PDE2 and PDE4 inhibition, delayed the fade of the response to 5-HT, while stimulation of soluble guanylyl cyclase independently of PDEs accelerated the fade of the response to 5-HT. In conclusion, both PDE3 and PDE4 subtypes are responsible for the suppression and the fade of the inotropic response to 5-HT and prucalopride. Signalling through the 5-HT(4) receptor remains fully active for at least 90 min with PDEs continuously regulating the response. cGMP levels, elevated by activation of particulate guanylyl cyclase under PDE2 inhibition, can indirectly enhance 5-HT(4) receptor-mediated signalling, at least when also PDE4 is inhibited, presumably through inhibition of PDE3. Elevation of cGMP generated by soluble guanylyl cyclase attenuates responses to 5-HT independently of PDEs.

The facilitating effect of prucalopride on cholinergic neurotransmission in pig gastric circular muscle is regulated by phosphodiesterase 4

The influence of the selective 5-HT(4) receptor agonist prucalopride on acetylcholine release from cholinergic nerve endings innervating pig gastric circular muscle and the possible regulation of this effect by phosphodiesterases (PDEs) was investigated, as PDEs have been shown to control the response to 5-HT(4) receptor activation in pig left atrium. Circular muscle strips were prepared from pig proximal stomach and either submaximal cholinergic contractions or tritium outflow after incubation with [(3)H]-choline, induced by electrical field stimulation, were studied. Prucalopride concentration-dependently increased the amplitude of submaximal cholinergic contractions and of acetylcholine release induced by electrical field stimulation. The effect of the highest concentration tested (0.3 μM) on cholinergic contractions was antagonized by the selective 5-HT(4) receptor antagonist GR113808 but not by granisetron or methysergide; the antagonism of prucalopride by GR113808 was confirmed in the release assay. The non-selective PDE-inhibitor 3-isobutyl-methyl-xanthine (IBMX) concentration-dependently reduced the amplitude of the cholinergic contractions; 3 μM IBMX reduced the cholinergic contractions maximally by 16% but it enhanced the facilitating effect of prucalopride from 51 to 83%. IBMX (10 μM) induced and enhanced the facilitating effect of prucalopride on electrically induced acetylcholine release. The selective inhibitors vinpocetine (PDE1), EHNA (PDE2) and cilostamide (PDE3) did not influence the effect of prucalopride on acetylcholine release but the PDE4-inhibitor rolipram (1 μM) enhanced the facilitating effect of prucalopride to the same extent as IBMX. These results demonstrate that 5-HT(4) receptors are present on the cholinergic nerves towards the pig gastric circular muscle, facilitating acetylcholine release; the intracellular transduction pathway of this facilitation is regulated by PDE4. Combination of a 5-HT(4) receptor agonist with selective inhibition of the PDE involved in this regulation of transmitter release might enhance the prokinetic effect of the 5-HT(4) receptor agonist.

Phosphodiesterases reduce spontaneous sinoatrial beating but not the 'fight or flight' tachycardia elicited by agonists through Gs-protein-coupled receptors

Cyclic AMP (cAMP) steers the generation of basal heart beat in the sinoatrial node. It also induces sinoatrial tachycardia and increased cardiac force, elicited through activation of Gs-protein-coupled receptors (GsPCRs). Phosphodiesterases (PDEs) hydrolyse cAMP. In the heart mainly PDE3 and PDE4 would be expected to limit those functions, and the PDE isoenzymes do indeed reduce basal sinoatrial beating rate and blunt the positive inotropic effects of agonists, mediated by GsPCRs. By contrast, recent evidence shows that GsPCR-mediated sinoatrial tachycardia is not controlled by PDE1-5. A PDE-resistant cAMP pool in sinoatrial cells, generated through activation of GsPCRs, including β(1)- and β(2)-adrenoceptors, appears to guarantee unrestrained tachycardia during fight or flight stress.

Conserved expression and functions of PDE4 in rodent and human heart

PDE4 isoenzymes are critical in the control of cAMP signaling in rodent cardiac myocytes. Ablation of PDE4 affects multiple key players in excitation–contraction coupling and predisposes mice to the development of heart failure. As little is known about PDE4 in human heart, we explored to what extent cardiac expression and functions of PDE4 are conserved between rodents and humans. We find considerable similarities including comparable amounts of PDE4 activity expressed, expression of the same PDE4 subtypes and splicing variants, anchoring of PDE4 to the same subcellular compartments and macromolecular signaling complexes, and downregulation of PDE4 activity and protein in heart failure. The major difference between the species is a fivefold higher amount of non-PDE4 activity in human hearts compared to rodents. As a consequence, the effect of PDE4 inactivation is different in rodents and humans. PDE4 inhibition leads to increased phosphorylation of virtually all PKA substrates in mouse cardiomyocytes, but increased phosphorylation of only a restricted number of proteins in human cardiomyocytes. Our findings suggest that PDE4s have a similar role in the local regulation of cAMP signaling in rodent and human heart. However, inhibition of PDE4 has ‘global’ effects on cAMP signaling only in rodent hearts, as PDE4 comprises a large fraction of the total cardiac PDE activity in rodents but not in humans. These differences may explain the distinct pharmacological effects of PDE4 inhibition in rodent and human hearts.

Cardiac overexpression of the human 5-HT4 receptor in mice

Serotonin (5-HT) exerts pleiotropic effects in the human cardiovascular system. Some of the effects are thought to be mediated via 5-HT(4) receptors, which are expressed in the human atrium and in ventricular tissue. However, a true animal model to study these receptors in more detail has been hitherto lacking. Therefore, we generated, for the first time, a transgenic (TG) mouse with cardiac myocyte-specific expression of the human 5-HT(4) receptor. RT-PCR and immunohistochemistry revealed expression of the receptor at the mRNA and protein levels. Stimulation of isolated cardiac preparations by isoproterenol increased phospholamban phosphorylation at Ser(16) and Thr(17) sites. 5-HT increased phosphorylation only in TG mice but not in wild-type (WT) mice. Furthermore, 5-HT increased contractility in isolated perfused hearts from TG mice but not WT mice. These effects of 5-HT could be blocked by the 5-HT(4) receptor-selective antagonist GR-125487. An intravenous infusion of 5-HT increased left ventricular contractility in TG mice but not in WT mice. Similarly, the increase in contractility by 5-HT in isolated cardiomyocytes from TG mice was accompanied by and probably mediated through an increase in L-type Ca(2+) channel current and in Ca(2+) transients. In intact animals, echocardiography revealed an inotropic and chronotropic effect of subcutaneously injected 5-HT in TG mice but not in WT mice. In isolated hearts from TG mice, spontaneous polymorphic atrial arrhythmias were noted. These findings demonstrate the functional expression of 5-HT(4) receptors in the heart of TG mice, and a potential proarrhythmic effect in the atrium. Therefore, 5-HT(4) receptor-expressing mice might be a useful model to mimic the human heart, where 5-HT(4) receptors are present and functional in the atrium and ventricle of the healthy and failing heart, and to investigate the influence of 5-HT in the development of cardiac arrhythmias and heart failure.

Function of cardiac beta1- and beta2-adrenoceptors of newborn piglets: role of phosphodiesterases PDE3 and PDE4

The structures of porcine and human beta(2)-adrenoceptors differ but the repercussions for porcine cardiac function are unknown. We investigated the function of porcine beta(2)-adrenoceptors in 3 cardiac regions, sinoatrial node, left atrium and right ventricle of newborn piglets. Both (-)-noradrenaline and (-)-adrenaline caused sinoatrial tachycardia: 60+/-10% and 62+/-7% of the maximum response (E(max)) to (-)-noradrenaline (-logEC(50)=9.0) and (-)-adrenaline (-logEC(50)=7.5) respectively, were resistant to antagonism by the beta(1)-selective CGP20712A (2-hydroxy-5-[2-[[2-hydroxy-3-[4-[1-methyl-4-(trifluoromethyl)-1H-imidazol-2-yl]phenoxy]propyl]amino]ethoxy]-benzamide) (300 nM) but antagonized by beta(2)-selective ICI118551 (erythro(+/-)-[1-(2,3-dihydro-7-methyl-1H-inden-4-yl)oxy]-3-[(1-methylethyl)amino]-2-butanol) (50 nM), consistent with mediation through beta(2)-adrenoceptors. The phosphodiesterase3-selective inhibitor cilostamide and phosphodiesterase4-selective inhibitor rolipram did not affect catecholamine chronotropic potencies. Only small CGP20712A-resistant positive inotropic effects of (-)-adrenaline were detected in the left atria (13+/-2% of E(max)) and ventricular trabeculae (14+/-5% of E(max)). The atrial inotropic responses to (-)-noradrenaline and (-)-adrenaline faded; fades were prevented by rolipram but not cilostamide or concurrent cilostamide+rolipram respectively. (-)-Noradrenaline (ICI118551 present) increased left atrial cAMP levels through beta(1)-adrenoceptors that were markedly enhanced by rolipram but unaffected by cilostamide. Concurrent cilostamide+rolipram uncovered inotropic and cAMP responses to (-)-adrenaline (CGP20712A present). We conclude that sinoatrial beta(2)-adrenoceptors are more important than beta(1)-adrenoceptors in the mediation of tachycardia caused by both (-)-noradrenaline and (-)-adrenaline in the newborn piglet. beta(2)-adrenoceptors have only a minor role in the mediation of left atrial and ventricular inotropic effects of (-)-adrenaline. Catecholamine-evoked tachycardia is not controlled by PDE3 or PDE4. PDE4, but not PDE3, controls the atrial inotropic and cAMP beta(1)-adrenoceptor-mediated responses to (-)-noradrenaline. Both PDE3 and PDE4 blunt left atrial inotropic and cAMP responses to (-)-adrenaline through beta(2)-adrenoceptors.

Phosphodiesterases do not limit beta1-adrenoceptor-mediated sinoatrial tachycardia: evidence with PDE3 and PDE4 in rabbits and PDE1-5 in rats

The mammalian heart expresses at least five phosphodiesterases (PDE1-5). Catecholamines produce surges of inotropically relevant cAMP through beta(1)-adrenoceptor stimulation. cAMP is mainly hydrolysed by PDE3 and/or PDE4 thereby blunting contractility. Basal sinoatrial beating rate in mouse, rat, piglet and rabbit sinoatrial cells is reduced by PDE3 and/or PDE4 through hydrolysis of cAMP. However, in rodents, the tachycardia elicited by catecholamines through production of cAMP by beta-adrenoceptor activation is not controlled by PDE3 and PDE4, despite a blunting effect of PDE3 or/and PDE4 on basal sinoatrial beating, but it is unknown whether PDE3 limits catecholamine-evoked tachycardia in the rabbit. Since rabbit sinoatrial cells are an important model for pacemaker research, we investigated whether the positive chronotropic effects of (-)-noradrenaline on spontaneously beating right atria of the rabbit are potentiated by inhibition of PDE3 with cilostamide (300 nM). We also studied the sinoatrial effects of the PDE4 inhibitor rolipram (10 microM) and its influence on the responses to (-)-noradrenaline. For comparison, we investigated the influence of cilostamide and rolipram on the positive inotropic responses to (-)-noradrenaline on rabbit left atria and right ventricular papillary muscles. Cilostamide and concurrent cilostamide + rolipram, but not rolipram alone, increased sinoatrial rate by 15% and 31% of the effect of (-)-isoprenaline (200 microM) but the PDE inhibitors did not significantly change the chronotropic potency of (-)-noradrenaline. In contrast in papillary muscle, the positive inotropic effects of (-)-noradrenaline were potentiated 2.4-, 2.6- and 44-fold by cilostamide, rolipram and concurrent cilostamide + rolipram, respectively. In left atrium, the positive inotropic effects of (-)-noradrenaline were marginally potentiated by cilostamide, as well as potentiated 2.7- and 32-fold by rolipram and by concurrent cilostamide and rolipram respectively. To compare the influence of PDE1-5 on basal sinoatrial rate and (-)-noradrenaline-evoked tachycardia, we investigated on rat right atria the effects of selective inhibitors. The PDE4 inhibitor rolipram and non-selective inhibitor isobutyl-methylxanthine caused tachycardia with -logEC(50)s of 7.2 and 5.0 and E(max) of 18% and 102% of (-)-isoprenaline, respectively. Rolipram did not change the chronotropic potency of (-)-noradrenaline. At high concentrations (10-30 microM), the PDE1, PDE3 and PDE5 inhibitors 8-methoxymethyl-3-isobutyl-1-methylxanthine, cilostamide and sildenafil, respectively, caused marginal tachycardia but did not significantly change the chronotropic potency of (-)-noradrenaline. The PDE2-selective inhibitor erythro-9-[2-hydroxy-3-nonyl]adenine caused marginal bradycardia at 30 microM and tended to reduce the chronotropic potency of (-)-noradrenaline. Rabbit PDE3 reduces basal sinoatrial rate. Although PDE4 only marginally reduces rate, under conditions of PDE3 inhibition, it further reduces sinoatrial rate. Both PDE3 and PDE4 control atrial and ventricular positive inotropic effects of (-)-noradrenaline. In contrast, neither PDE3 nor PDE4 limit the sinoatrial tachycardia induced by (-)-noradrenaline. In the rat, only PDE4, but not PDE1, PDE2, PDE3 and PDE5, reduces basal sinoatrial rate. None of the five rat PDEs limits the (-)-noradrenaline-evoked tachycardia. Taken together, these results confirm and expand evidence for our proposal that the cAMP-compartment modulating basal sinoatrial rate, controlled by PDE3 and/or PDE4, is different from the PDE-resistant cAMP compartment involved in beta(1)-adrenoceptor-mediated sinoatrial tachycardia.

Phosphodiesterases PDE3 and PDE4 jointly control the inotropic effects but not chronotropic effects of (-)-CGP12177 despite PDE4-evoked sinoatrial bradycardia in rat atrium

Acting through a low-affinity site of the beta(1)-adrenoceptor (beta(1L)AR), CGP12177 causes sinoatrial tachycardia and positive inotropic effects in left atrium but not in the ventricle of the rat. However, inhibition of either PDE3 or PDE4 also uncovers positive inotropic effects of CGP12177 in ventricle, but whether these phosphodiesterases also control the atrial agonist effects of CGP12177 was unknown. We, therefore, investigated the effects of the PDE3-selective inhibitor cilostamide (300 nM) and PDE4 inhibitor rolipram (1 microM) on the (-)-CGP12177-evoked increases of sinoatrial beating rate and force of paced left atria of the rat. Rolipram (n = 8) increased basal sinoatrial rate by 27 +/- 5 bpm but cilostamide (n = 8) had no effect. The chronotropic potency of (-)-CGP12177 (-logEC(50)M = 7.5) was not changed by rolipram and cilostamide or their combination. (-)-CGP12177 increased left atrial force with intrinsic activity 0.25 compared to (-)-isoprenaline. Rolipram (n = 8) and cilostamide (n = 8) did not change basal force of left atria but concurrent rolipram + cilostamide (n = 8) increased force by 52 +/- 9% of the effect of 200 microM (-)-isoprenaline. Neither rolipram nor cilostamide affected the inotropic potency of (-)-CGP12177 (-logEC(50)M = 7.4) but concurrent rolipram + cilostamide caused potentiation (-logEC(50)M = 8.2) and converted (-)-CGP12177 into a full agonist compared to (-)-isoprenaline. Cyclic AMP appears to maintain sinoatrial rate and PDE4 elicits bradycardia through hydrolysis of cAMP in a compartment distinct from the beta(1L)AR-induced cAMP compartment through which (-)-CGP12177 causes tachycardia. In contrast to the (-)-CGP12177-evoked tachycardia, not controlled by PDE3 and PDE4, these isoenzymes jointly reduce (-)-CGP12177-evoked increases of left atrial contractility through beta(1L)AR.